Youxi Lin

Metamorphic InAsSb/AlInAsSb heterostructures for optoelectronic applications

Metamorphic heterostructures containing bulk InAs1−xSbx layers and AlInAsSb barriers were grown on GaSb substrates. The lattice mismatch (up to 2.1%) between the GaSb substrates and the InAsSb layers was accommodated by the growth of GaInSb linearly graded buffers. The 1 μm thick InAsSb0.44 layer with an absorption edge above 9 μm exhibited an in-plane residual strain of about 0.08%. InAs1−xSbx structures with x = 0.2 and x = 0.44 operated as light emitting diodes at 80 K demonstrated output powers of 90 μW and 8 μW at 5 μm and 8 μm, respectively.

Effects of carrier concentration and phonon energy on carrier lifetime in Type-2 SLS and properties of InAs 1-X Sb X alloys

GaInSb and AlGaInSb compositionally graded buffer layers grown on GaSb by MBE were used to develop unrelaxed InAs1-XSbXepilayers with lattice constants up to 2.1 % larger than that of GaSb. The InAsSb buffer layer was used to grow InAs0.12Sb0.88 layer on InSb. The structural and optical characterization of 1-μm thick InAs1-xSbx layers was performed together with measurements of the carrier lifetime.

Metamorphic InAsSb-based barrier photodetectors for the long wave infrared region

InAs0.6Sb0.4/Al0.75In0.25Sb-based barrier photodetectors were grown metamorphically on compositionally graded Ga1−xInxSb buffer layers and GaSb substrates by molecular beam epitaxy. At the wavelength of 8 μm and T = 150 K, devices with 1-μm thick absorbers demonstrated an external quantum efficiency of 18% under a bias voltage of 0.45 V.

Interband absorption strength in long-wave infrared type-II superlattices with small and large superlattice periods compared to bulk materials

The absorption spectra for the antimonide-based type-II superlattices (SLs) for detection in the long-wave infrared (LWIR) are calculated and compared to the measured data for SLs and bulk materials with the same energy gap (HgCdTe and InAsSb). We include the results for the metamorphic InAsSbx/InAsSby SLs with small periods as well as the more conventional strain-balanced InAs/Ga(In)Sb and InAs/InAsSb SLs on GaSb substrates. The absorption strength in small-period metamorphic SLs is similar to the bulk materials, while the SLs with an average lattice constant matched to GaSb have significantly lower absorption. This is because the electron-hole overlap in the strain-balanced type-II LWIR SLs occurs primarily in the hole well, which constitutes a relatively small fraction of the total thickness.

Unrelaxed bulk InAsSb with novel absorption, carrier transport, and recombination properties for MWIR and LWIR photodetectors

The optical properties of bulk unrelaxed InAsSb layers having a low temperature photoluminescence (PL) peak up to 10 μm are presented. The materials were grown on GaSb substrates by molecular beam epitaxy. The lattice mismatch between the epilayers and GaSb substrates was accommodated with linearly graded GaAlInSb buffers. An 11-meV width of PL at full-width half-maximum was measured for InAsSb with Sb compositions of 20 and 44% . The best fit for the dependence of the energy gap on Sb composition was obtained with a 0.9-eV bowing parameter. Temperature dependences of the energy gap for InAsSb alloys with 20 % and 44% Sb were determined from PL spectra in the temperature range from 12 to 300 K. A T=77 K minority carrier lifetime up to 350 ns in undoped InAsSb layers with 20% Sb was determined from PL kinetics.

Background and interface electron populations in InAs0.58Sb0.42

The unintentional background electron population and associated interface and surface conductivity in a heterostructure of InAs0.58Sb0.42 with a bandgap of 0.144 eV and AlInSb was studied with multi-carrier Hall-effect analysis. A free electron bulk concentration at 77 K was found with a density of 2.4 × 1015 cm−3and mobility of 140 000 cm2 V−1 s−1. A surface electron accumulation layer was observed with a density of 5.5 × 1011 cm−2 and mobility of 4500 cm2 V−1 s−1 that is consistent with predictions of surface Fermi level pinning. Another accumulation layer was identified at the interface with the AlInSb of 4 × 1011 cm−2 with a mobility of 37 000 cm2 V−1 s−1. The origin of the defects and the implications for device structures are discussed.

Electronic properties of unstrained unrelaxed narrow gap InAs x Sb1−x alloys

The electronic properties of unstrained unrelaxed InAs x Sb1−x alloys have been determined in a wide range of alloy compositions using IR magnetospectroscopy, magnetotransport and IR photoluminescence. All studied alloys have n-type background doping with electron concentration decreasing with the Sb content. The composition dependence of the background doping concentration follows an empirical exponential law in a wide range of compositions. Both bandgap and electron effective mass dependence on alloy composition exhibit negative bowing reaching lowest values at x = 0.63: E g = 0.10 eV, m* = 0.0082 m 0 at 4.2 K. The bowing coefficient of 0.038 m 0 obtained for the electron effective mass is in good agreement with that obtained from the Kane model.

Extremely small bandgaps, engineered by controlled multi-scale ordering in InAsSb

The relationship between the effective bandgap and the crystalline structure in ordered InAsSb material has been studied. Modulation of the As/Sb ratio was induced along the growth direction during molecular beam epitaxy, producing a strained layer superlattice. To enable the use of concentration ratios near unity in both layers in the period, the structures were grown with negligible net strain on a virtual substrate with a lattice constant considerably larger than that of GaSb. The bandgap line-up of InAsSb layers with different compositions is such that a type II superlattice is formed, which exhibits smaller bandgaps than either of the two constituents. It can also be smaller than the possible minimum direct-bandgap of the alloy. From observations of CuPt ordering in bulk layers with small amounts of strain of both signs, we postulate that strain is the main driving force for atomic ordering in InAsSb. Because the modulated structures exhibit small but opposing amounts of strain, both layers in the period exhibit ordering at the atomic scale throughout the structure. Since the strain can be controlled, the ordering can be controlled and sustained for arbitrary thick layers, unlike the situation in uniform bulk layers where the residual strain eventually leads to dislocation formation. This offers a unique way of using ordering at two different scales to engineer the band-structure.

Transport properties of holes in bulk InAsSb and performance of barrier long-wavelength infrared detectors

A new method for determination of minority carrier lifetimes, mobilities and diffusion lengths is demonstrated. From the transient response of long wavelength infrared barrier detectors with a cut-off of 10 micron, the mobility of the minority holes and their lifetime in bulk InAs0.6Sb0.4 at T = 77 K was determined to be 800 cm2 V−1s−1 and 165 ns, respectively.

Infrared emitters and photodetectors with InAsSb bulk active region

Bulk unrelaxed InAsSb alloys with Sb compositions up to 44 % and layer thicknesses up to 3 µm were grown by molecular beam epitaxy. The alloys showed photoluminescence (PL) energies as low as 0.12 eV at T = 13 K. The electroluminescence and quantum efficiency data demonstrated with unoptimized barrier heterostructures at T= 80 and 150 K suggested large absorption and carrier lifetimes sufficient for the development of long wave infrared detectors and emitters with high quantum efficiency. The minority hole transport was found to be adequate for development of the detectors and emitters with large active layer thickness.